Heretofore bulk memories for computer systems have frequently utilized magnetic core technology wherein a large number of magnetic rings are employed with each ring serving as a memory storage element. The data state of the ring is dependent upon the direction of magnetization of the ring. A primary advantage of this type memory is that the information stored in the memory is not lost when power is removed. The rings remain magnetized in the selected states even when power is not supplied to the memory unit. A magnetic core memory can be reactivated and returned to use immediately upon reapplication of electrical power. There is no need to reload the programs and data into memory each time the power is disconnected.
In large computer systems, programs and data are frequently stored on disk memories so that the system can be initialized from disk, even after a power failure. But in smaller computer applications, programs and data are often entered manually and are not stored in any readily accessible manner. Therefore, a power loss which causes loss of the stored bit pattern in memory is a serious failure of the system which cannot be remedied by merely restoring main power.
There have recently come into widespread use semiconductor random access memories which have decided advantages over the older core type memories. In particular, the newer memories are faster, have lower power consumption and occupy less space. However, a serious drawback in the use of semiconductor memories is that each of the memory element circuits is volatile, that is, the information stored in the memory element is lost when power is removed from the memory circuit. With such a memory system the programs and data stored in the memory are lost whenever power is removed from the memory unit. Although the loss of power does not result in circuit damage, the loss of stored information does require that the system be reloaded with programs and data before processing can continue. The reloading of programs is a time consuming process which reduces the efficient utilization of a computer system. In certain types of systems a provision is made to transfer the contents of memory to disk when a failure is first indicated. But in many computer systems a power failure occurs so rapidly that all of the memory contents cannot be transferred to disk. This is particularly true for process control systems.
It has been proposed in using static semiconductor memory systems that backup power be automatically supplied to the memory circuit upon detection of failure of the primary power. When the memory circuit is receiving backup power the bit lines which transfer data into and from the memory cells is permitted to go to a low state. In this low state it is very possible that the individual memory cells can be discharged due to subthreshold conduction if the memory stays in the backup mode for a prolonged period of time. Further when power is eventually restored there will be a substantial capacitive coupling of the substrate of the memory due to the capacitance between the bit lines, other internal nodes and the substrate. It is likely that a substantial transient voltage applied to the substrate will cause data to be effected in the memory cells.
Therefore, there exists a need for a circuit for protecting the bit pattern stored in the memory cells of a static memory when the memory itself is connected to operate on a backup power source after failure of the main power supply.